1. Field of the Invention
This invention generally relates to electrochemical storage devices and, more particularly, to a system and method for the large scale fabrication of metal cyanometallates.
2. Description of the Related Art
Large-capacity, cost-effective energy storage is the transformational technology needed to enable the large scale integration of renewable energy sources, such as wind and solar. The rechargeable battery offers efficient electrical energy storage (EES). The Li-ion battery has been the leading option based on its performance, but the source materials are too expensive for large-scale EES. Therefore, superior low cost, high-performance electrode materials are required to compete against the high cost of lithium-ion batteries, or a new system of rechargeable metal-ion batteries must be developed to replace lithium-ion batteries.
Sodium/potassium-ion batteries have recently received a great deal of attention because the natural reserves of sodium/potassium in the crust of the earth are much higher than lithium. This abundance makes possible the development of low cost batteries for EES. However, it has proved impractical to mimic the structures of Li+-host compounds for Na+ or K+-host compounds. Sodium/potassium ions are much larger than lithium ions and they severely distort the structures of the Li+-host compounds. Thus, for the development of sodium/potassium-ion batteries it is important to develop new Na+/K+-host materials with large interstitial spaces in which sodium/potassium-ions can easily and reversibly move. Transition metal cyanometallate (TMCM) materials with large interstitial spaces have been investigated as cathode materials for rechargeable lithium-ion batteries [1, 2], sodium-ion batteries [3, 4], and potassium-ion batteries [5].
With an aqueous electrolyte containing alkali-ions or ammonium-ions, copper and nickel hexacyanoferrates [(Cu,Ni)—HCFs] have demonstrated a robust cycling life with 83% capacity retention after 40,000 cycles at a charge/discharge current of 17C [6-8]. In spite of this, the materials demonstrated low capacities and energy densities because (1) only one sodium-ion can be inserted/extracted reversibly into/from Cu—HCF or Ni—HCF per formula unit and (2) these TM-HCFs electrodes must be operated below 1.23 V due to the water electrochemical window. In order to compensate for such shortcomings, manganese hexacyanoferrate (Mn—HCF) and iron hexacyanoferrate (Fe—HCF) were used as cathode materials in non-aqueous electrolyte [9, 10]. When assembled with a sodium-metal anode, Mn—HCF and Fe—HCF electrodes cycled between 2.0 volts (V) and 4.2 V delivered capacities ˜110 milliamp hours per gram (mAh/g). More recently, FeFe(CN)6.4H2O and Na0.61Fe[Fe(CN)6]0.94 were reported to exhibit high energy density and power density and good stability during cycling [11, 12].
FIG. 1 is a diagram depicting the open framework of a metal cyanometallate (prior art). All the compounds mentioned above can be categorized as metal cyanometallates (MCMs) with the general formula AxM1mM2nCN)z that constructs an open framework as shown. The open framework structure of the transition metal MCMs facilitates both rapid and reversible intercalation processes for alkali and alkaline ions (Ax). The MCMs capacity is determined by the available A-sites in the compounds into which the alkali or alkaline ions can be inserted reversibly in the range of working voltages. From the electric neutrality point of view, the valences of M1 and M2 mainly contribute to the amount of the available A-sites. For example, 2 sodium-ions can be inserted/extracted into/from Na2MnFe(CN)6 between 2-4 V vs. Na° because the valences of Mn- and Fe-ions can change between +2 and +3, which provides a theoretical capacity of 171 mAh/g. Noteworthy is the fact that as many as possible metal-ions, “A”, should exist in the MCMs to deliver a high capacity in a metal-ion battery with a non-metal counter electrode. In addition, in order to neutralize charges, the transition metals should be kept at low valances.
It would be advantageous if the synthesis of MCMs could be optimized, with improvements to increase MCM particle sizes, and able to operate at a large scale for mass production.    [1] V. D. Neff, Some performance characteristics of a Prussian Blue battery, Journal of Electrochemical Society, 132 (1985) 1382-1384.    [2] N. Imanishi, T. Morikawa, J. Kondo, Y. Takeda, O. Yamamoto, N. Kinugasa, T. Yamagishi, Lithium intercalation behavior into iron cyanide complex as positive electrode of lithium secondary battery, Journal of Power Sources, 79 (1999) 215-219.    [3] Y. Lu, L. Wang, J. Cheng, J. B. Goodenough, Prussian blue: a new framework for sodium batteries, Chemistry Communication, 48(2012)6544-6546.    [4] L. Wang, Y. Lu, J. Liu, M. Xu, J. Cheng, D. Zhang, J. B. Goodenough, A superior low-cost cathode for a Na-ion battery, Angew. Chem. Int. Ed., 52(2013)1964-1967.    [5] A. Eftekhari, Potassium secondary cell based on Prussian blue cathode, J. Power Sources, 126 (2004) 221-228.    [6] C. D. Wessells, R. A. Huggins, Y. Cui, Copper hexacyanoferrate battery electrodes with long cycle life and high power, Nature Communication, 2(2011) 550.    [7] C. D. Wessells, S. V. Peddada, R. A. Huggins, Y. Cui, Nickel hexacyanoferrate nanoparticle electrodes for aqueous sodium and potassium ion batteries, Nano Letter, 11(2011) 5421-5425.    [8] C. D. Wessells, S. V. Peddada, M. T. McDowell, R. A. Huggins, Y. Cui, The effect of insertion species on nanostructured open framework hexacyanoferrate battery electrode, J. Electrochem. Soc., 159(2012) A98-A103.    [9] T. Matsuda, M. Takachi, Y. Moritomo, A sodium manganese ferrocyanide thin film for Na-ion batteries, Chemical Communications, DOI: 10.1039/C3CC38839E.    [10] S.-H. Yu, M. Shokouhimehr, T. Hyeon, Y.-E. Sung, Iron hexacyanoferrate nanoparticles as cathode materials for lithium and sodium rechargeable batteries, ECS Electrochemistry Letters, 2(2013)A39-A41.    [11] X. Wu, W. Den, J. Qian, Y. Cao, X. Ai, H. Yang, Single-crystal FeFe(CN)6 nanoparticles: a high capacity and high rate cathode for Na-ion batteries, J. Mater. Chem. A., 1(2013)10130-10134.    [12] Y.-G. Guo, Y. You, X.-L. Wu, Y.-X. Yin, High-quality Prussian blue crystals as superior cathode materials for room-temperature sodium-ion batteries, Energy & Environmental Science, (2014) DOI: 10.1039/C3EE44004D